The present invention relates to a graphic data processing apparatus for removing the jags of edges included in vector data and, more particularly, to a graphic data processing apparatus for determining the tones (densities) of edge pixels of vector data and feeding the determined tones to a laser printer or similar output unit.
It is a common practice with computer graphics to execute antialiasing which allows an image to appear more attractive on a CRT (Cathode Ray Tube). Conventional approaches for implementing antialiasing include (1) an uniform averaging method, (2) a weighted averaging method (2) and (3) a convolutional integration method.
With the advance of so-called DTP (Desk Top Publishing) using a personal computer, systems of the type printing out vector images similar to those which are handled by the computer graphics art are extensively used today. Typical of such systems is one using Postscript of Adbi. Postscript belongs to a family of languages usually referred to as PDLs (Page Description Languages). PDL is a programming language for describing a form representative of the contents of one document including the text, graphics, and their arrangement and format. Regarding a character font, this type of system is implemented with a vector font. Hence, even when the text is changed in magnification, this system prints it out in far higher quality than a system which uses a bit map font (e.g. conventional word processor). Another advantage particular to the above-stated system is that both the character font and the graphics can be printed out in combination.
However, a laser printer applicable to such a system has a resolution which is not higher than 240 dpi to 400 dpi and, like CRT of computer graphics, suffers from alias. It is, therefore, necessary to provide even the laser printer type printing with an antialias implementation so as to produce high quality images.
When a conventional graphic data processing apparatus is implemented with antialiasing which uses N.times.N subpixel devision, it cannot achieve sufficient effects since it uses only moderate N from the standpoint of processing time and subjective evaluation of image quality. Specifically, excessively great N would increase the calculating time while excessively small N would limit the effect.
A conventional graphic data processing apparatus using the uniform averaging scheme uses only one kind of subpixel configuration such as N * M submatix in calculating the tone (luminance and density) of a pixel. This brings about a problem that, depending on the inclination of vector data, the tone produced from the actual image and the tone produced from the subpixel configuration greatly differ from each other, resulting in insufficient antialiasing.
A graphic data processing apparatus implemented by the weighted averaging scheme or the convolutional integration scheme is advantageous over the apparatus implemented by the uniform averaging scheme since it reduces the difference between the tone derived from the actual area and the tone derived from the subpixel configuration and thereby enhancing the antialiasing effect. Such an apparatus, however, consumes a substantial period of time in calculating the area ratio and, therefore, slows down the processing.
Moreover, in a graphic data processing apparatus provided with any one of the conventional antialiasing schemes, despite that a laser printer which outputs an image by electrophotographic process is substituted for CRT which plays the role of an output unit, the luminance of CRT is simply replaced with the density of the laser printer. This is undesirable since the characteristic particular to an electrophotographic process prevents the antialiasing effect from being fully exhibited.